I downloaded and installed a plain Drupal and Busy. Activated busy and changed something in style.css. It works right away. Are you using aggregated css files? If so clear your caches. It's actually easy - you check the path to style.css and if you change something in that file and it doesn't show up then it must be a Cache problem. I doubt this is a theme problem.

I was having the same problem. I cleared the cache multiple times and nothing changed. I unchecked aggregating CSS and cleared the cache, nothing changed. I noticed that the color.css file that was being used was in default/files/color/color.css. Drupal 7 seems to move it there. I went back to the global settings on the appearance page and just clicked on Save configuration even though I didn't change anything, and that made Drupal add the changed color.css to the default/files/color/ location.
I don't get why this is set up this way. It took a long time to figure this out over several weeks. I had been going directly to the default/files/color/color.cc and making changes, but I think that will eventually be overwritten when I save configuration again.

So I had to do this again. I cleared the cache, unchecked and then re-checked the compress and aggregate files and cleared the cache in between. What worked was saving the configuration on the appearance page of the theme I am using.